1. Field of the Invention
This invention relates to the field of switched-mode power supplies (SMPS), and particularly to methods of determining the current flowing in the inductor of an SMPS.
2. Description of the Related Art
Switched-mode power supplies (SMPS), which switch voltage to and from an inductor to effect current through it (e.g., to provide a regulated voltage output), often use information about the current (IL) flowing in the inductor to control the switching. One way of sensing IL is by adding a current-sensing resistor in series with the inductor; the voltage across the resistor varies with IL. However, the use of a suitable current-sensing resistor adds cost to the SMPS, and power is dissipated as heat in the resistor.
Minimizing the size of the current-sensing resistor lessens these problems: a lower resistance reduces power dissipation, allowing the use of a physically smaller resistor which lowers cost. This approach also has drawbacks, however. With a lower resistance, a smaller voltage is developed across the resistor, which results in a low signal-to-noise ratio (SNR). Minimizing the resistance value also has the effect of increasing the relative impedance of the resistor's unavoidable parasitic inductance. This can cause the voltage across the resistor to become distorted with respect to the sensed current. Analog and digital filtering techniques have been used to mitigate these problems, but these increase cost and complexity.
Another technique for producing a signal which varies with IL involves emulating the inductor current using an RC integrator. A resistor and capacitor are connected in series to form an integrator, which is connected in parallel across the SMPS' inductor. When the resistor and capacitor are properly chosen, the voltage across the capacitor emulates the inductor current. The accuracy of this approach is optimized over a wide bandwidth when the RC integrator's time constant is matched to the time constant of the inductor and its equivalent series resistance.
Unfortunately, this emulation method has several problems. The equivalent series resistance of the inductor may not be well-controlled or specified, and can vary substantially over temperature and process. This results in a loss of accuracy. If the time constants are not well-matched, the emulation circuit can suffer a loss of bandwidth, and the emulated current signal can become distorted. The mismatch of time constants shows up in the emulated current signal as an exponential decay of the average of the emulated current signal toward the average inductor current times the equivalent series resistance of the inductor.
This approach can also result in a poor SNR: the magnitude of the emulated current is given by the inductor current multiplied by the inductor's equivalent series resistance—which is preferably made as small as possible to minimize power dissipation in the inductor. However, a small equivalent series resistance results in a small emulated current signal, and thus a poor SNR.